This invention generally relates to stress-engineered metal films, and more particularly to photo lithographically patterned spring structures formed from stress-engineered metal films.
Photo lithographically patterned spring structures (sometimes referred to as xe2x80x9cmicro-springsxe2x80x9d) have been developed, for example, to produce low cost probe cards, and to provide electrical connections between integrated circuits. A typical spring includes a spring metal finger having a flat anchor portion secured to a substrate, and a curved claw extending from the anchor portion and bending away from the substrate. The spring metal finger is formed from a stress-engineered metal film (i.e., a metal film fabricated such that its lower portions have a higher internal compressive stress than its upper portions) that is at least partially formed on a release material layer. The claw of the spring metal finger bends away from the substrate when the release material located under the claw is etched away. The internal stress gradient is produced in the spring metal by layering different metals having the desired stress characteristics, or using a single metal by altering the fabrication parameters. Such spring metal structures may be used in probe cards, for electrically bonding integrated circuits, circuit boards, and electrode arrays, and for producing other devices such as inductors, variable capacitors, and actuated mirrors. For example, when utilized in a probe card application, the tip of the claw is brought into contact with a contact pad formed on an integrated circuit, and signals are passed between the integrated circuit and test equipment via the probe card (i.e., using the spring metal structure as a conductor). Other examples of such spring structures are disclosed in U.S. Pat. No. 3,842,189 (Southgate) and U.S. Pat. No. 5,613,861 (Smith).
The present inventors recognized that most failures of spring structures (e.g., separation of the spring structure from an underlying substrate through delamination or peeling) occur a significant amount of time after fabrication. The present inventors believe these failures are caused at least in part by the internal stress gradient retained in the anchor portion of the spring metal finger. That is, although the internal stress is essentially relieved in the claw of the spring metal finger upon release, the internal stress is retained in the anchor portion of the spring metal finger, along with other xe2x80x9ctracexe2x80x9d or unreleased portions of the spring metal layer. Over time, this retained internal stress is believed to bend the edges of the anchor portion upward (i.e., away from the underlying substrate), thereby causing delamination or peeling that weakens the attachment of the spring metal finger to the substrate. It is essential that the unlifted anchor portion of the spring metal finger adheres to the substrate (i.e., that the anchor portion resists the internal stress tending to bend the edges of the anchor portion away from the substrate). Most probing and packaging applications require large amounts of contact force (xcx9c50-100 mg) between the claw tip and a contacted structure. The force scales quadratically with film thickness, but the peeling moment increases also.
One possible solution to the delamination/peeling problem is to use a spring material in which the stress is annealed out after release (i.e., after the claw of the spring metal finger is allowed to bend away from the substrate). However, this solution places other limitations on the material properties, such as a reduction in the total stress differental.
Another solution is to incorporate a ductile, dry etchable metal such as Aluminum (Al) or Titanium (Ti) as an interfacial release layer between the substrate and the finger metal. This approach has been demonstrated to improve adherence of the anchor portion to the substrate when the thickness and/or internal stress of the spring metal layer is relatively small, but is less effective as the thickness or the stress of the metal layer is increased.
What is needed is a spring structure that resists delamination and/or peeling, thereby improving the strength and durability of the spring structures.
In accordance with the present invention, the strength and durability of a spring structure is increased by providing a stress-balancing pad formed on the unlifted anchor portion of the spring metal finger, where the stress-balancing pad is formed with an internal stress gradient (and stress moment) that is opposite in sign to the internal stress gradient (and stress moment) of the spring metal finger. Specifically, in contrast to the spring metal finger, the stress-balancing pad is formed from a stress-engineered metal film fabricated such that portions furthest from the anchor portion have a higher internal compressive stress than portions closest to the anchor portion. This opposite internal stress gradient causes the stress-balancing pad to apply a downward force on the edges of the anchor portion, thereby resisting the delamination or peeling of the anchor portion that can result in separation from an underlying substrate. In one embodiment, the internal stress gradient (and moment) of the stress-balancing pad has a magnitude that is equal to or greater than the internal stress gradient (and moment) of the spring metal finger, thereby preventing delamination or peeling of the anchor portion by completely countering (nullifying) the internal stress (and moment) of the spring metal finger.
In accordance with an aspect of the present invention, the spring metal finger and the stress-balancing pad can be formed either from materials that have the same composition, or from materials that have different compositions. For example, both the spring metal finger and the stress-balancing pad can be formed from Mo or MoCr. The fabrication process is simplified when the same material is used for both layers because the number of targets in the deposition equipment is minimized. However, an etch stop layer (e.g., Cr or Ti) may be needed between the spring metal finger and the stress-balancing pad to prevent undesirable etching of the spring metal finger during the fabrication process. When different materials are used, it may be necessary to increase the number of deposition equipment targets, but the etch stop layer can be omitted when the two materials are selectively etchable. For example, a stress-balancing pad formed from Mo is selectively etched from a spring metal finger formed from MoCr using an anisotropic fluorine etch. Similarly, a stress-balancing pad formed from Ti solution hardened with Si (Ti:Si) is selectively removed from a spring metal finger formed from NiZr using a Ti etch. Note that the stress-balancing pad can be electrically conducting or non-conducting, but electrical conductivity of the stress-balancing pad beneficially improves the total conductance through the anchor portion of the spring metal finger, and through other trace structures formed on the substrate using the spring metal and stress-balancing layers.
In accordance with another aspect of the present invention, the spring structure further includes a support pad formed between the anchor portion of the spring metal finger and the substrate. When formed from a conductive material (e.g., Ti), the support pad may be utilized to conduct signals between the spring metal finger and a conductor formed on the substrate under the support pad. In one embodiment, the support pad is formed from a portion of the release material layer.
In accordance with yet another aspect of the present invention, a spring structure is fabricated by forming a spring metal island on a release material island, forming the stress-balancing pad over an anchor portion of the spring metal island, and then releasing the claw portion of the spring metal finger by removing an associated portion of the release material island.
In accordance with a first disclosed method, a release material layer, a spring metal layer, and a stress-balancing layer are sequentially deposited and then etched using a first mask to form the spring metal and release material islands. In the first method, a stress-balancing island is formed that completely covers the spring metal island. A release mask is then used both to remove a portion of the stress-balancing island located over the claw portion of the spring metal island, thereby forming the stress-balancing pad on the anchor portion, and to etch the release material located under the claw portion of the spring metal island. A portion of the release material is utilized to form the support pad under the anchor portion. The first method minimizes the number of fabrication steps, but typically requires the use of different material compositions to form the spring metal layer and the stress-balancing layer.
In accordance with a second disclosed method both the spring metal and stress-balancing layers are formed from the same material composition, but requires an intervening etch stop layer. The second method is otherwise similar to the first method in that both the spring metal layer and the stress-balancing layer (along with the intervening etch stop layer) are deposited/grown before the spring metal mask is used to pattern the spring metal and stress-balancing islands. The second method may require more processing time than the first method, but reduces the number of targets needed in the deposition equipment, thereby potentially reducing deposition system overhead associated with process and control calibration.
Similar to the second method, a third disclosed method facilitates forming the spring metal finger and the stress-balancing pad using the same material composition, but avoids the need for an etch stop layer by utilizing a special mask to lift off pattern the stress-balancing pad onto the anchor portion of the spring metal finger. In particular, the release material layer and a spring metal layer are sequentially deposited and then etched using a first mask to form the spring metal and release material islands. A second mask is then used that exposes the anchor portion of the spring metal island, but covers the claw portion. A stress-balancing layer is then deposited which forms the stress-balancing pad on the anchor portion when the second mask is lifted off. A release mask is then used to etch release material located under the claw portion to release the claw. Although fabrication costs are increased because three masks are required, the third method provides the benefits associated with using the same material composition for both the spring metal finger and the stress-balancing pad without requiring an intervening etch stop layer. If desired, the mask count can be reduced by using the stress balancing pad to define the release window, but this approach may modify the design rules undesirably.
Similar to the third method, fourth possible method also utilizes three masks to form the spring metal finger, but the stress balancing pad is formed before the spring metal island is etched. In particular, a release material layer, a spring metal layer, and a stress balancing layer are sequentially deposited. A first mask is then used to etch only the stress balancing layer, thereby forming the stress balancing pad. The spring metal layer and release layer are then etched using a second mask to form the spring metal and release material islands. A release mask is then used to etch release material located under the claw portion to release the claw.